The present invention relates to communications systems and methods, and more particularly, to systems and methods for diversity communication in wireless communications systems.
As known to those skilled in the art, a cellular wireless communications system typically includes a plurality of base stations that support radio communications with wireless terminals, e.g., vehicle-mounted or handheld cellphones or other communications devices, over a plurality of geographical areas referred to as “cells.” Typically, when a terminal begins to move to the fringe of a first cell, toward a second cell, it attempts to establish communications with the base station serving the second cell in order to continue the call or other communications session in which it is currently engaged.
Such a procedure, commonly referred to as a “handoff,” may be performed in a number of different ways. For example, in many conventional systems, a “hard handoff” procedure is performed wherein a terminal ends communications with a first base station before resuming communications with a second base station. Although such a procedure can be effective, it may be vulnerable to “call drops” or other interruptions during the attempted handoff.
Systems providing code division multiple access (CDMA) using channels defined by spreading codes commonly offer a capability, referred to as “soft handoff,” in which a terminal can transition from a first base station to a second base station using an intermediate stage in which the terminal communicates simultaneously with both base stations. Such a capability can arise because CDMA terminals and base station equipment typically use a RAKE receiver architecture that allows signals carrying the same information but transmitted by multiple base stations over different signal paths to be commonly processed to achieve the benefits of spatial diversity. Soft handoff procedures for IS-95 CDMA are described in Section 6.6 of Mobile Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems, TIA/EIA-95-B (October 1998).
This capability of conventional CDMA receivers may also be used for purposes other than soft handoff. For example, simultaneous transmission from multiple base stations may be used to improve signal reception at a terminal, thus allowing the transmit power from each base station to be reduced. This can reduce interference and, thus, can increase available capacity in a wireless communications system.
Newly proposed CDMA standards typically are designed to be backward compatible with older CDMA standards, such that systems implementing a new standard are capable of supporting terminals designed to conform to an older standard. For example, the new IS-2000 specification includes radio configurations, i.e., combinations of spreading and other channel codes, that are also included in the IS-95 specification, as well as newer, e.g., higher data rate, radio configurations that are not supported under the IS-95 specification.